The present invention is an improvement of the system described in U.S. Pat. No. 5,469,742 ("Acoustic Temperature and Film Thickness Monitor and Method"). A goal of that prior art system was to monitor the temperature and/or thickness of a material during a semiconductor processing step without influencing the processing and with good survivability. The pin transducers described therein include an acoustic transducer at one end and a sharpened tip at the other end designed to contact a material to be monitored. In this system the transducer excites acoustic energy in the pin that is coupled by the tip into the material as Lamb or other acoustic waves. By monitoring the velocity of the Lamb waves in the material the thickness and/or temperature of the material can be determined (Lamb wave velocity varies with material thickness and temperature). The prior art system indirectly determines the Lamb wave velocity by measuring the time of flight of the Lamb waves between the (transmitting) pin that excited the Lamb waves and at least one receiving pin contacting the material.
This system measures the time between the occurrence of a predetermined zero crossing in the analog signal generated by an echo on the transmitting pin and the occurrence of pre-determined zero-crossing in the analog signal generated by the receiving transducer upon receiving acoustic energy excited in the receiving pin by a passing Lamb wave. The travel time of the acoustic energy in one of the pins is subtracted from the total time, yielding the time of flight between the transmitting pin and the particular receiving pin. Problems with this system include the need for sophisticated analog electronics to perform the necessary signal processing and errors due to differences between the pins.
Another problem with the prior art system described above and many other systems that use ultrasonic energy to monitor material characteristics is that they typically provide relative temperature and/or thickness measurements. I.e., these systems determine the change in a parameter and cannot absolutely determine a material parameter in practical situations.